Data Quality

Executive Summary: The DATA flag indicates if data is available from GWOSC. In general, a search for
binary black hole events will analyze data that passes the DATA and the tests called CBC_CAT1 and CBC_CAT2.

LIGO data quality categories, or flags, are defined by each analysis group: Compact Binary Coalescence (CBC), Burst, Continuous Waves (CW) and Stochastic. This is because periods of noisy data will affect each type of analysis differently.
For each flag, GWOSC data files contain a corresponding 1 Hz time-series that marks times
that pass the flag as a "1" (good data), and times that fail the flag as a "0" (bad data).
A full list of O1 data quality categories can be seen on the
O1 data quality definitions page.
The details of each category
are described in the references linked below. However, as a rough guide:

DATA (Data Available): Failing this level indicates that LIGO data are not publicly available because the instruments were not operating in an acceptable condition. For O1, this is equivalent to Category 1.

CAT1 (Category 1): Failing a data quality check at this category indicates a critical issue with a key detector component not operating in its nominal configuration. Since these times indicate a major known problem these times are identical for each data analysis group.
Times that fail CAT1 flags are not available as LIGO open data -- not available at this web site.

CAT2 (Category 2): Failing a data quality check at this category indicates times when there is a known, understood physical coupling to the gravitational wave channel. This might include times of high seismic activity.

CAT3 (Category 3): Failing a data quality check at this category indicates times when there is statistical coupling to the gravitational wave channel which is not fully understood.

In general, data quality levels are defined in a cumulative way: a time
which fails a given category automatically fails all higher
categories. For example, if the only known problem with a given time
fails a burst category 2 flag, then the data is said to pass DATA and
BURST_CAT1, but fails BURST_CAT2 and BURST_CAT3.
However, the different analysis groups are independent: if something fails at CAT2_BURST, then it can pass CAT2_CBC.

These graduated categories of quality allow a data pipeline to adjust its behavior
depending on the data quality. For example running the numerical search (template matching)
against all the data segments that pass CAT1, but ignoring any candidate events
from data that do not pass CAT3. This strategy allows long sections of data to be used, increasing
search efficiency.

Note to LSC members: Conventionally, hardware injections are vetoed by CAT flags so that
searches do not see them. However GWOSC strain data provides h(t) at these times: therefore a
search with GWOSC data will find lots of chirps, to be compared with the lists of injections -- see below.

Hardware Injections

The O1 data set contains simulated astrophysical signals, known as hardware injections, used for testing and calibration.
For an example, see the Find a Hardware Injection Tutorial.
For complete documentation, see:

Instrumental Spectral Lines

A power spectral density of LIGO data typically shows a number of spectral lines. Many of these are associated with known instrumental resonances.
The O1 instrumental spectral lines page gives an explanation, and catalog, of
these instrumental lines.

Representative PSDs

Technical Details

GWOSC 4KHz strain data have been repackaged and downsampled from 16384 Hz to 4096 Hz.
Advanced LIGO data are not calibrated or valid below 10 Hz or above 5 kHz, and the data sampled at 4096 Hz are not valid above 2 kHz. In most searches for astrophysical sources, data below 20 Hz are not used because the noise is too high.
More detailed information about the data set can be seen on the Technical Details page.